Mobile communication devices typified by smartphones and mobile phones have been sophisticated. To deal with increased volume of data and increased speed in communications, communication standards have been increasingly sophisticated. In frequency bands for wireless communications, a gap between a transmit band and a receive band has narrowed, and the prevention of the interference with the neighboring band has been highly required. Such circumstances require filters and duplexers to have more sophisticated characteristics. For example, to achieve a duplexer having a wide passband and a narrow gap between bands, desired is an acoustic wave device having a low-loss, a large electromechanical coupling coefficient, and excellent temperature characteristics, which means a Temperature Coefficient of Frequency (TCF) close to zero.
Japanese Patent Application Publication No. 2013-055371 (Patent Document 1) discloses a technique that forms a silicon oxide film doped with an element so that the silicon oxide film doped with the element covers a comb-shaped electrode. Japanese Patent Application Publication Nos. 2009-201168 and 2012-149310 (Patent Documents 2 and 3) disclose techniques that form a silicon oxide film by sputtering so that the silicon oxide film covers the comb-shaped electrode.
The silicon oxide film formed so as to cover a comb-shaped electrode can bring the TCF close to zero, and improve the temperature characteristics. However, none of Patent Documents 1 through 3 discloses a technique that allows a resonator to have good temperature characteristics, and improves the characteristics such as a Q-value and an electromechanical coupling coefficient k2 of the resonator.